1. Field of the Invention
The present invention relates to the broad field of flue gas desulfurization. Flue gas desulfurization, as it applies to the present invention, generally relates to the removal of sulfur dioxide (SO.sub.2) from flue gases produced by fossil-fueled boilers used to generate electric power, process steam, etc. The Environmental Protection Agency promulgates regulations which require SO.sub.2 removal devices or other means for controlling SO.sub.2 emissions on many such flue gas producers, i.e., those that fire boilers with coal, oil, or other types of fossil fuel. The assignee of the present invention is, of course, a major producer of electric power, a portion of which is produced by the burning of fossil fuel, and has a major test facility at its Shawnee Power Plant wherein is conducted research in this field on technologies concerned specifically, but not exclusively, with desulfurization of flue gas from coal-fired power generation stations.
2. Description of the Prior Art
As was mentioned earlier, at the inception of utilizing lime and/or limestone scrubbing facilities for flue gas desulfurization, the scrubber wastes were initially disposed of by incorporating the slurry resulting therefrom in ponds. This procedure, which is termed by some as ponding, has proven to be environmentally unattractive because upon closure of the ponds, the acreage is completely unfit for any commerically attractive subsequent industrial or agricultural use thereof. As an alternate to this ponding approach, the method of dewatering the initial limestone scrubber slurry was turned to as a possible viable alternative. Following this approach, a substantial amount of the water resulting from the limestone scrubbing would be removed by various means from the scrubber slurry to increase thereby the solids content thereof and provide for disposal of same as solid landfill. This approach appears to be the most practical disposal method at present.
Accordingly, this approach dictates that the scrubber slurry be dewatered to a point wherein the solids resulting therefrom, usually in the form of filter cake, can be used directly as landfill. Furthermore, due to the large tonnages of material produced as waste from these processes, it goes without saying that transportation of same away from the production site thereof would greatly add to the cost of the overall operation. Accordingly, not only must the slurry be dewatered to the point wherein the resulting product is suitable for landfill, it must be in a form suitable for such purposes at or close to the site wherein it is produced. As was noted earlier, the sulfite sludges resulting from the throwaway lime or limestone scrubbing operations do not lend themselves to effective or efficient dewatering operations by conventional means. Such waste products generally cannot be dewatered directly to solids contents ranging greater than about 60 to 70 percent. Such materials exhibit thixotropic properties or can return to such state upon wetting and if they are not further treated but rather are placed directly in landfills at or near the plant site, they do not meet the criteria of being both economically and environmentally attractive. A technical solution to this problem is the approach of further processing such materials by forcing copious quantities of air into admixture therewith to effect oxidation therein of the sulfite to the sulfate form prior to the dewatering operation. Such a forced oxidation approach renders the resulting calcuim sulfate crystals in the sludge slurry generally satisfactory as far as the dewatering characteristics thereof; however, both the capital investment necessary for compressor capacity and the energy required to operate the compressors greatly adds to the cost of this approach.
Still another approach to rendering the sulfite sludge products usable for landfill is by the physical stabilization thereof which may be accomplished by effectively reducing the moisture content of the sludge to the point where the structural properties thereof are optimized when the material is disposed of in landfill. This may be accomplished by the addition thereto of relatively dry materials such as, for example, fly ash, soil, or other materials. This approach accomplishes at least two things: (1) it dries out the mixture by spreading entrained water throughout a larger weight of solids while modifying the particle-size distribution so that closer packing can be accomplished; and (2) the additional drying and closer packing usually result in increased shear strength, lower permeability, and lower combined volume of the two materials, which, in the case of fly ash and sludge, are both waste materials seeking disposal solutions. However, there are some major drawbacks to this approach, the principal one, perhaps, being that since minimal chemical reactions are involved, the stabilization process, to wit, adding a dry material to the slurry is, of course, reversible and if the mixture is subsequently rewetted, for instance by heavy rains, and allowed to saturate, the fluidization thereof will usually cause rapid decreases in shear strength leading to probable failure of structures supported thereupon and possible flow of the resulting uncontained sludge. In addition, insufficient fly ash may be available.
Yet another approach is chemical stabilization in which specific chemical binders or cementitious materials are added to the sludge. This binder addition, however, adds considerable expense and is not currently used extensively in the industry.
It should be readily apparent, of course, that the foregoing brief description of some of the prior art is perfunctory at best since voluminous periodicals and reports thereupon have been written, edited, and rewritten on the whole general subject area. Those skilled in the art interested in further pursuing such subject matter might avail themselves with copies of any of such numerous treatises including, for example, FGD Sludge Disposal Manual, Second Edition, of the Electric Power Research Institute dated September 1980.